The present invention relates to an arrangement in connection with a piston engine provided with a turbocompressor.
The invention also relates to a method in connection with a piston engine provided with a turbocompressor.
It is known in the art of engine technology to use turbocompressors for increasing the engine output. A turbocompressor comprises a compressor section, by which pressurized combustion air is introduced into the engine. In addition, the turbocompressor comprises a turbine section for driving the compressor. The exhaust gases coming from the engine are led to the turbine, which converts the energy of the exhaust gases into driving power for the compressor. Carbon deposit is formed on those parts of the turbine, which come into contact with exhaust gas. Carbon build-up is intensive especially when heavy fuel oil is used as a fuel. The temperature of the turbine surfaces coming into contact with exhaust gas increases substantially, whereby the carbon deposit built on the surfaces is especially hard. Carbon build-up is particularly problematic in the region between the turbine blades and the flow duct around it, as well as in the nozzle ring located prior to the turbine blades in the flow direction of the exhaust gas. The qualities of the carbon deposit vary according to the composition of the used fuel. The problem is significant especially with heavy fuel oils containing vanadium and sodium. The carbon deposit formed on the turbine reduces the efficiency of the turbine and consequently the engine output decreases. Therefore, the contamination deposit formed on the turbine has to be removed regularly.
For cleaning the turbine, the engine may be stopped and a mechanical cleaning performed, whereby the turbocharger is dismantled and the turbine cleaned. This is, however, a very disadvantageous procedure, as it is very time-consuming. Besides, the contamination is often so heavy that this method would call for shutdowns more often than the service of the engine itself would require.
Several solutions have been developed for removing carbon deposits from the turbine surfaces, while the turbine remains in operation. In general, these can be divided into wet and dry methods, in which the cleaning is performed either by means of water injection or by mixing granular solid matter with the exhaust gases in order to clean the surfaces mechanically. One disadvantage associated with the dry methods is the simultaneous wear of the surfaces of the device to be cleaned. The effect of water injection may be either like washing, when the engine loads are low, or like a thermal shock, when used at a fairly high engine load. When the cleaning type is washing, the cleaning effect is based first on the mechanical impact of water drops and secondly, on the dissolution of the contaminant in water. Typically, this cleaning by washing requires a reduction of engine output to 30-40% of the maximum output. Cleaning, which involves a thermal shock, is, instead, based on the different thermal expansion qualities of the contamination deposit and the adherence surface as well as on the separation effect accomplished by sudden cooling. Cleaning based on a thermal shock may be performed with higher engine output than cleaning by washing.
The publication U.S. Pat. No. 5,944,483 discloses one wet cleaning method based on a thermal shock. In the method, relatively small amounts of water are injected repeatedly into the turbine so that the water strikes the nozzle ring in liquid state at a sufficiently low temperature and causes a thermal shock in the deposits. The nozzle ring is allowed to reheat between the injections back to its normal operating temperature. The device described in the publication is also applicable for turbine cleaning by washing.
During wet cleaning the exhaust gases coming to the turbine need to be at a sufficiently low temperature in order to avoid excessive vaporization of water and damage to the turbine components on account of a thermal shock. If the engine is operated at too high output, the exhaust gases entering the turbine may be too hot for wet cleaning, whereby the engine output has to be decreased for the cleaning operation. The length of the cleaning sequence for a turbine is typically about half an hour and for engines, which are in continuous operation and use fuel of poor quality, the sequence is run through as often as twice a week. The reduction in electricity production caused by the turbine cleaning, e.g. in the case of engines at power plants, may cause severe economical losses.
The object of the present invention is to provide a technical solution, by which a piston engine provided with a turbocompressor may be operated at a higher output during the cleaning of the turbine.